This invention relates to a color cathode ray tube, and particularly, to a color cathode ray tube having an in-line electron gun for radiating three co-planar beams and to a self-convergence system in which rasters formed on a phosphor screen by the three beams have an equal size, under a common deflecting magnetic field.
The three co-planar beams of an in-line electron gun are deflected horizontally and vertically by a deflection yoke disposed on a funneled part of a glass envelope, to form rasters on a phosphor screen. To work the color cathode ray tube on a self-convergence system whereby a dynamic convergence correction is not required, a coma distortion is minimized by adjusting a horizontal deflecting magnetic field of the deflection yoke to give a strong pincushion distortion and a vertical deflecting magnetic field to give a strong barrel distortion, thus forming an accordant raster on the phosphor screen. In this case, however, the raster scanned by the central beam of the three beams is generally smaller in both horizontal and vertical dimensions, than the rasters formed by each of the outside beams. A mismatching of the rasters is due to a coma distortion of the deflection yoke. In order to attain a coincidence of the rasters by removing the coma distortion, a field control element consisting of a high permeability magnetic member is disposed on the bottom of a shield cup formed in a bottomed cylinder with a non-magnetic material which is mounted on a tip of the electron gun to which a rear leakage magnetic field of the deflection yoke is exerted.
Recently, a color display tube with a high resolution characteristic has been employed for display of various data, thereby giving alphanumeric character, symbol, Chinese characters, diagram, etc. in high density.
For a high density display, it is necessary for a resolution of the color cathode ray tube to be high; a focusing characteristic to be uniform; and a frequency band of a video signal circuit to be wide to improve a horizontal resolution of the displayed picture. Many scanning lines are required to improve the a vertical resolution.
To increase the number of scanning lines as an available means for high density display, a horizontal deflecting frequency f.sub.h is enhanced to a value higher than the 15.734 KHz which is used in the current standard TV system. In this case, however, a coma distortion arises on the rasters formed by the central beam and by the beams on both sides according to a horizontal deflecting field which was not observed at the horizontal deflecting frequency f.sub.h =15.734 KHz. Thus, a problem is quite unavoidable because a grade of the picture displayed on a phosphor screen is severely deteriorated thereby.